Question

What is the difference between the electron-domain geometry and the molecular geometry of a molecule? Use the water molecule as an example in your discussion.

Short answer

The electron-domain geometry of a molecule refers to the arrangement of electron domains around the central atom, considering both bonding and non-bonding electron domains, while molecular geometry focuses on the arrangement of atoms and only takes into account bonding electron domains. For water (H2O), the electron-domain geometry is tetrahedral because of the four electron domains around the central oxygen atom (two single bonds to hydrogen and two lone pairs). However, the molecular geometry is bent or angular because the lone pairs on the oxygen atom cause a deviation from the ideal tetrahedral structure, pushing the hydrogen atoms closer together.

Step-by-step solution

Define Electron-Domain Geometry and Molecular Geometry

Electron-domain geometry refers to the arrangement of electron domains around the central atom in a molecule. An electron domain is any area where there are electrons present; this can be a single bond, a double bond, a triple bond, or a lone pair of electrons. Electron-domain geometry considers both bonding and non-bonding electron domains to determine the geometrical shape. Molecular geometry, on the other hand, is the arrangement of atoms in a molecule. This determines the actual shape of the molecule. Molecular geometry takes into account only the bonding electron domains, meaning non-bonding electron domains (lone pairs) are not considered when determining the shape of the molecule.

Understand the VSEPR Theory

The valence-shell electron-pair repulsion (VSEPR) theory is used to predict the geometry of molecules based on their electron-domain geometry. The main idea of VSEPR theory is that electron domains will try to arrange themselves around the central atom to minimize repulsion between negative charges, which will dictate the final molecular geometry.

Determine the Electron-Domain Geometry of the Water Molecule

The water molecule (H2O) is composed of one oxygen atom centrally bonded to two hydrogen atoms. In addition to these bonds, the oxygen atom has two lone pairs of electrons. So, we have a total of four electron domains around the central oxygen atom (two single bonds and two lone pairs). According to the VSEPR theory, these four electron domains will arrange themselves tetrahedrally around the oxygen atom to minimize repulsion. Therefore, the electron-domain geometry of the water molecule is tetrahedral.

Determine the Molecular Geometry of the Water Molecule

To find the molecular geometry, we only consider the bonding electron domains. In the case of the water molecule, there are only two bonding electron domains, which are the single bonds between the oxygen atom and the two hydrogen atoms. Since the electron-domain geometry is tetrahedral, but we are only considering the bonding electron domains, the molecular geometry will be a bent or angular shape. This is because the two lone pairs on the oxygen atom cause a deviation from the ideal tetrahedral structure and push the hydrogen atoms closer together, thus creating a bent shape. In conclusion, the electron-domain geometry of the water molecule is tetrahedral due to the presence of four electron domains around the central oxygen atom, whereas the molecular geometry is bent due to the influence of the lone pairs on the bonding electron domains. Understanding both electron-domain geometry and molecular geometry helps to determine the true shape and properties of a molecule.

Key concepts

VSEPR Theory

Valence-shell electron-pair repulsion (VSEPR) theory is helpful to predict the structure of molecules. It is based on the principle that electron domains (regions where electrons are most likely to be found, such as bonds and lone pairs) will repel each other and therefore, will organize themselves as far apart as possible around a central atom.

According to VSEPR theory, the shape adopted by the molecule minimizes the repulsion between these electron domains. This principle is crucial for determining both the electron-domain geometry (which includes both bonding and non-bonding electrons) and the molecular geometry (which considers only bonding electrons).

Understanding VSEPR theory is essential because the shape of a molecule greatly affects its physical and chemical properties.

Lone Pair of Electrons

A lone pair of electrons refers to a pair of valence electrons that are not shared with another atom and do not participate in bonding. These electron pairs can greatly influence the geometry of molecules. They occupy more space around the central atom than the bonding pairs, leading to more significant repulsion.

It's important to note that while lone pairs affect the overall shape of a molecule, they are not included when naming the molecular geometry. During the process of determining the shape of a molecule, lone pairs will be part of the initial electron-domain geometry but are then 'invisible' when we name the molecular geometry.

Tetrahedral Geometry

Tetrahedral geometry is one of the basic shapes described by VSEPR theory. It occurs when there are four electron domains surrounding a central atom. Imagine taking a central point and placing four other points around it so that they are as far apart as possible; they would form a tetrahedron.

Each point represents an area where electrons are likely to be found, which could be either a bond to another atom or a lone pair. When a central atom has four bonding electron domains and no lone pairs, it adopts a perfect tetrahedral shape. However, if there are lone pairs, they can distort the geometry due to the extra space they take up and the repulsion they cause.

Bent Molecular Shape

The bent or angular molecular shape is a type of molecular geometry that results when there are lone pairs pushing away bonding pairs of electrons. In the water molecule, for instance, the oxygen atom has two pairs of bonding electrons (attached to hydrogen atoms) and two lone pairs.

While the electron-domain geometry considers all four pairs and gives a tetrahedral arrangement, the lone pairs are not counted in the molecular geometry. Thus, the actual shape 'bends' due to the repulsion from the lone pairs, resulting in an angular molecular geometry. This concept is crucial when predicting how molecules with similar electron-domain geometries can have very different molecular shapes due to the presence of lone pairs.